Hematologic malignancies are clonal neoplastic disorders arising from hematopoietic stem cells or committed progenitor cells in the bone marrow. They encompass a diverse group of diseases classified by cell lineage (myeloid vs lymphoid) and clinical behavior (acute vs chronic). The laboratory plays a central role in diagnosis through the CBC, peripheral blood smear, bone marrow examination, flow cytometry, cytogenetics, and molecular testing.
Classification Framework
The World Health Organization (WHO) classification integrates clinical features, morphology, immunophenotype, genetics, and molecular markers. The major categories include: myeloproliferative neoplasms (MPN, e.g., CML, PV, ET, PMF), myelodysplastic syndromes (MDS), myelodysplastic/myeloproliferative overlap neoplasms (MDS/MPN, e.g., CMML), acute myeloid leukemia (AML) and related neoplasms, precursor lymphoid neoplasms (ALL/LBL), mature B-cell neoplasms (CLL/SLL, DLBCL, follicular lymphoma, multiple myeloma, Hodgkin lymphoma), and mature T-cell and NK-cell neoplasms.
Diagnostic Approach
The initial evaluation begins with a complete blood count and differential. Abnormalities may include cytopenias (anemia, thrombocytopenia, neutropenia) due to bone marrow failure, or cytoses (leukocytosis, thrombocytosis, erythrocytosis) indicating a proliferative process. Blasts — immature cells with high nuclear-to-cytoplasmic ratio, open chromatin, prominent nucleoli, and basophilic cytoplasm — are the hallmark of acute leukemia. The peripheral blood smear is examined for blasts, abnormal WBC morphology (dysplastic neutrophils, Pelger-Huët anomaly, abnormal monocytes), and abnormal RBC morphology (teardrop cells in myelofibrosis, nucleated RBCs). Thrombocytopenia or thrombocytosis may also be present.
Bone Marrow Aspiration and Biopsy
Bone marrow aspiration and biopsy are essential for definitive diagnosis. The aspirate provides cells for morphologic assessment (differential count, blast percentage, dysplasia), flow cytometry (immunophenotyping), cytogenetic analysis (karyotype, FISH), and molecular studies (PCR, sequencing). The biopsy (trephine core) provides architecture: cellularity, fibrosis, infiltration pattern, and granulomas. Blast count > 20% in blood or bone marrow defines acute leukemia per WHO criteria (except for AML with recurrent genetic abnormalities). Aspirate dry tap suggests myelofibrosis or packed marrow.
Flow Cytometry
Flow cytometry is critical for lineage assignment and classification. Cells are stained with fluorochrome-conjugated antibodies against cluster of differentiation (CD) antigens. Myeloid markers: CD13, CD33, CD117, myeloperoxidase (MPO), CD11c, CD14, CD64, CD15. Monocytic markers: CD14, CD64, CD11c, CD36. Erythroid markers: CD235a (glycophorin A), CD71. Megakaryocytic markers: CD41 (GPIIb), CD61 (GPIIIa), CD42b. B-lymphoid markers: CD19, CD20, CD22, CD79a, PAX5, surface immunoglobulin. T-lymphoid markers: CD2, CD3, CD5, CD7, CD4, CD8. Natural killer (NK) cell markers: CD16, CD56, CD57. Acute leukemia is further classified by lineage: acute myeloid leukemia (AML), B-acute lymphoblastic leukemia (B-ALL), and T-acute lymphoblastic leukemia (T-ALL). Mixed-phenotype acute leukemia (MPAL) expresses markers of more than one lineage.
Cytogenetics and Molecular Genetics
Chromosomal abnormalities are defining features of many hematologic malignancies and carry important prognostic information. Recurrent translocations in AML include t(8;21)(q22;q22) RUNX1-RUNX1T1, inv(16)(p13.1q22) CBFB-MYH11, t(15;17)(q24;q21) PML-RARA (acute promyelocytic leukemia), and t(9;11)(p22;q23) KMT2A-MLLT3. In ALL, common abnormalities include hyperdiploidy, t(12;21) ETV6-RUNX1, t(9;22) BCR-ABL1 (Ph-positive ALL), and KMT2A rearrangements. In CML, t(9;22) BCR-ABL1 is pathognomonic. The Philadelphia chromosome (Ph) t(9;22) is also seen in some cases of ALL and rarely AML. Fluorescence in situ hybridization (FISH) detects specific rearrangements, while karyotyping provides a genome-wide view. Molecular testing by PCR and next-generation sequencing identifies mutations in genes such as NPM1, FLT3-ITD, CEBPA, RUNX1, TP53, IDH1/2, DNMT3A, ASXL1, and JAK2.
Minimal Residual Disease (MRD) Monitoring
MRD refers to the presence of leukemic cells below the morphologic detection threshold (typically < 5% blasts). MRD is the strongest predictor of relapse in acute leukemias. Detection methods include multiparameter flow cytometry (sensitivity 10⁻⁴ to 10⁻⁵), PCR amplification of fusion transcripts (sensitivity 10⁻⁵ to 10⁻⁶), and next-generation sequencing of clonal rearrangements (sensitivity 10⁻⁶). Serial MRD monitoring guides treatment intensification, stem cell transplantation timing, and early detection of impending relapse.
